Primary radiographic images are conventionally created by exposing a black and white photographic film or plate to an X-ray source and an interposed sample. The more absorbent parts of the sample throw shadows onto the photographic film or plate, and they appear less dark when the film or plate is developed. A radiograph of the human body, for example, shows the bones whiter than surrounding flesh because bones contain the element calcium, which has a relatively high atomic number. Abnormalities and foreign bodies are readily visible, and appropriate therapeutic action can be taken. Internal organs generally absorb X-rays to about the same extent as the surrounding flesh, but they can be shown up on a radiograph by concentrating material of greater absorbing power into the organ.
Radiography also has important industrial uses in locating internal defects in materials in creating three-dimensional images through stereoscopy, and in studying the three-dimensional structure of solids through tomography.
In all these applications, the exposed photographic film or plate is developed and employed diagnostically in medical applications and for quality control, record-keeping and scientific investigation in medical, industrial and scientific applications. Depending on the exposure radiation, the nature of the sample and the photochemistry of the photographic medium and its processing, results in a film radiograph that may possess a continuous tonal gradation in transmissivity to light extending between fully transparent (light) and fully opaque (black). The accurate reproduction of copies of the developed image is dependent on the ability of the techniques employed to faithfully reproduce the gray level gradation between the black and white extremes in the original continuous tone radiographic image.
Direct copying of the image has typically been attempted by photographic and xerographic techniques which rely upon exposure of a second photographic medium or xerographic drum to light transmitted through or reflected by the original image. Losses in tonal density and balance may occur, particularly in the xerographic reproduction process.
More recently, radiographic film images have been scanned by laser light beam scanners and photocell detectors to develop a digitized image field of the tonal density of the original image and to store the digitized image for transmission to remote locations and/or subsequent reproduction of the image. The digitization and storage of the image field also provides a back-up to the original film media which may be lost, particularly if it is sent to another location to be viewed by specialists in the field of interest.
The digital capture of diagnostic images from pre-existing radiographic film media typically employs laser light beam scanning systems of the type that are employed for either reading out or exposing or printing images. Scanning optical readers usually operate by detecting the light reflected from an illuminated spot on the incident surface of an opaque image bearing media. In the radiographic image context described above, where the image information is represented by the transmissivity of the media to light, the detector photo cell and optical system is arranged to detect the intensity of the light beam transmitted through the exit surface of the media rather than reflected by it. Such an optical scanner for reading out X-ray film images is disclosed in U.S. Pat. No. 4,818,861 where a laser beam light source is deflected through a predetermined generally flat scan angle in a scanning line across the radiographic image and the light transmitted by the radiographic image at each scanned point in each scanned line is collected and directed to a photodetector. These generally described scanner features are common to radiographic image scanning and image digitizing systems where the scanning beam is deflected across the nominal width of the radiographic image film media. Conventionally, the media is oriented in a plane perpendicular to the deflected light beam in both the scan and cross-scan directions at the center point of the scanning line. Thus, the light beam is deflected through the flat scan angle onto the incident surface of the media, and light transmitted through the media exits the exit surface and is collected by the collector optical system and presented to the photodetector as shown, for example, in the drawings of the '861 patent.
Typically, the constant intensity laser light beam is focused and deflected in the scan direction by optical systems of the type more specifically depicted in commonly assigned U.S. Pat. No. 4,921,320 to create a narrow band of light beam rays approximating a flat planar array to minimize the angulation of the light beam to the incident surface at the extreme end points of the scanning line.
The beam itself is typically circular in cross-section, although it has been proposed to employ an elliptical cross-section light beam having major and minor axis, through the use of suitably configured scanning deflectors, as taught, for example, in U.S. Pat. No. 4,943,128. The application of such circular or elliptical cross-section light beams to transparent image bearing media, such as radiographic film images, oriented perpendicularly to the scanning beam gives rise to the generation of interference of the directly transmitted incident light at the incident surface and the internally reflected light at the exiting surface of the media which appear as bands or contours at differing densities in the image, particularly as the media thickness varies. This effect is described in Section 7.5 of Principles in Optics by M. Born and E. Wolf, particularly FIG. 7.26. (In fact, this phenomena is used to interferometrically monitor the thickness of webs of manufactured film bases.) Such interference in the overlapping of the directly transmitted and the internally reflected light beams can give rise to significant modulation of transmitted intensity and introduces an image artifact causing a loss of accuracy in the read-out image.
Attempts have been made to reduce contouring and the overlap of incident directly transmitted scanning beams with light internally reflected back, especially at normal angles of incidence as reflected in the scanning systems proposed in Japanese Patent Laid-Open Numbers 63136-873A and 63136-874A. In one embodiment, it is suggested that the scanned medium be oriented at an angle other than 90.degree. to the planar angle of the scanning beam that traverses the document.